JP-A-2005-308380 (corresponding to U.S. Pat. No. 7,178,359) discloses a refrigerant cycle device in which a branch portion for branching the flow of refrigerant is provided on the upstream side of a nozzle portion of an ejector and on the downstream side of a radiator. A part of the refrigerant branched flows toward the nozzle portion, while the other refrigerant flows toward a refrigerant suction port of the ejector.
In the refrigerant cycle device as disclosed in JP-A-2005-308380, a first evaporator is disposed on the downstream side of a diffuser of the ejector, and a throttle mechanism and a second evaporator are disposed in a branch passage for connecting the branch portion with the refrigerant suction port. Both evaporators allow the refrigerant to obtain the effect of heat absorption.
However, the ejector to be applied to such a refrigerant cycle device expands the refrigerant isotropically at the nozzle portion, thereby recovering the loss of kinetic energy in expansion. This energy recovered (hereinafter referred to as “recovered energy”) is converted into pressure energy by the diffuser so as to decrease a driving force of a compressor, thereby improving a cycle efficiency (COP) of the refrigerant cycle device.
An ejector efficiency ηe indicative of energy conversion efficiency of the ejector is defined by the following formula (1):ηe=(1+Ge/Gnoz)×(ΔP/ρ)/ΔH  (1)
wherein Ge is the flow rate of the refrigerant sucked from the refrigerant suction port of the ejector, Gnoz is the flow rate of the refrigerant passing through the nozzle portion of the ejector, ΔP is the amount of an increase in pressure by the diffuser of the ejector, ρ is the density of the refrigerant sucked from the refrigerant suction port, and ΔH is a difference in enthalpy between an inlet and an outlet of the nozzle portion.
Even if the dimension, shape, and the like of each component of the ejector are designed such that the ejector efficiency ηe becomes a desired value, as represented by the above-mentioned formula (1), the absolute amount of ΔP/ρ, which is an index representing the pressure energy converted by the diffuser, cannot be increased unless the absolute amount of the enthalpy difference ΔH, which is an index representing the recovered energy by the nozzle portion, is increased.
That is, unless the absolute amount of the enthalpy is increased, the absolute amount of an increase in pressure ΔP cannot be increased at the predetermined ejector efficiency ηe. Accordingly, it is impossible to enhance the effect of improvement in the cycle efficiency (COP) by the increase in pressure of the refrigerant sucked into the compressor.